Fiber-optic probes make it possible to collect optical information such as Raman spectra without having to place the material being characterized inside a spectrometer housing. Such probes therefore simplify the interfacing of spectroscopic systems to chemical processes, and allow analytical instruments to be located remotely from hostile environments in need of spectroscopic monitoring.
The first remote fiber optic probes for Raman spectroscopy were reported by the McCreery group in the early 1980's. Their design used a single optical fiber to deliver laser light to the sample and a single optical fiber to collect light scattered by the sample. More specifically, divergent laser light from the laser delivery fiber was used to illuminate the sample, and light scattered from the sample within the acceptance cone of the collection fiber was transmitted back to the spectrograph. The efficiency of exciting and collecting Raman photons from any individual point in the sample was poor, but the integrated Raman intensity over the unusually large analysis volume compared favorably with the more traditional imaged illumination and collection.
McCreery's dual fiber Raman probe offered important benefits for remote and routine spectroscopy: 1) the sample could be distant from the Raman instrument, 2) no sample alignment was necessary once the probe was aligned to the spectrograph, 3) the probe could be less 1mm in diameter, making Raman measurements possible for samples with limited accessibility, 4) the probe could be placed directly in hostile samples (corrosive, hot, etc.) since only silica and the encapsulation material were exposed, and 5) multiple measurements could be made simultaneously by placing multiple collection fibers along the slit height of the spectrograph.
Several improvements to the McCreery Raman probe have more recently been reported. Instead of using just one collection fiber, multiple fibers have been used to increase the collection efficiency. For example, 6fibers, each having the same diameter as the excitation fiber, may be grouped around the excitation fiber to form a single circular layer, as shown in U.S. Pat. No. 4,573,761. Eighteen fibers, each having the same diameter as the excitation fiber, may also be grouped around the excitation fiber as two circular layers, and so on, though successive layers tend to be less effective at collecting Raman photons than the first layer.
The performance of the McCreery type probe can also be modified for improved collection efficiency and/or working distance by changing the overlap between the emission cone of the excitation fiber and the collection cones of the collection fibers. The first realization of this idea, reported by Leugers et al, angled the collection fibers such that their optic axes intersected the optic axis of the illumination fiber. This increased the overlap of the excitation and collection cones close to the tip of the fiber probe, where the excitation and collection of Raman photons was most efficient. The same concept was later implemented in a different way by O'Rourke and Livingston, who ground the tip of the probe into a cone shape, as discussed in U.S. Pat. No. 5,402,508. This shape was equivalent to putting prisms (or more correctly, axicon sections) on the collection fibers so that their optic axes crossed the optic axis of the excitation fiber.
One further variation of the McCreery probe design is to use collection fibers having a different diameter than the excitation fiber. This additional variable is useful for changing the working distance of the probe and the fiber coupling to the spectrograph. It is unclear how this changes the collection efficiency, however.
A serious problem with all of the fiber optic Raman robes discussed so far is probe background. Laser light inside an optical fiber generates Raman and fluorescence emission from the fiber core itself. This emission can overwhelm the desired Raman signal from the sample, and is the reason why the intense excitation light must be carried by a fiber separate from the collection fiber(s). Nevertheless, such emissions generated in the excitation fiber can result in unwanted reflection from the sample into the collection cones of the collection fibers. Furthermore, laser light reflected from the sample into the collection cones of the collection fibers may generate unwanted emission while propagating through the collection fibers back to the spectrograph. Symmetry arguments predict that the optical fiber emission observed at the spectrograph will have equal contributions from the excitation fiber and the collection fibers if the length of the excitation fiber is the same as the length of the collection fibers.
When a McCreery-style probe is inserted into a transparent solution, or into a homogeneous, partially absorbing solution, reflection of laser light or fiber emission back to the collection fibers is negligible and the optical fiber emission is not observed. Rather, it is the Raman spectra of scattering samples which tends to suffer from probe background. The impact of this background can sometimes be reduced to acceptable levels by angling the probe with respect to the sample surface normal, by eliminating the spectral region containing the fiber emission from the analysis, or by spectral subtraction of the fiber emission.
For many Raman applications elimination of the optical fiber background is at least desirable, if not necessary. The O'Rourke group and others have inserted a thin-film dielectric interference filter into the excitation fiber to filter out some of the optical fiber background. A technique to carry this out is to place the filter inside an SMA-to-SMA connector and put the fibers in contact with the filter. Another way to accomplish this is to collimate the laser output from a fiber, send the collimated beam through the filter, and focus the transmitted light back onto the other fiber. Both approaches require precise alignment and attenuate the laser energy. More importantly, the laser energy must still travel through several centimeters to several meters of fiber between the filter and the probe tip, depending on geometry constraints of the specific application. Emission from this intermediate length of fiber optic cable remained a serious problem.
In-line filters can also be added to the collection fibers, but the same limitations apply. In addition, the use of separate filters on each collection fiber rapidly becomes impractical. The use of a single filter for the collection fiber bundle requires precise rotational alignment, in addition to precise translational alignment. One other approach, coupling the collection fiber bundle to a single large fiber prior to filtering, seriously reduces optical throughput because the larger fiber couples more poorly to the associated spectrograph(s).
A more effective approach for eliminating optical fiber background is described in U.S. Pat. No. 5,112,127, wherein an optical probehead located between the sample and the optical fibers is used to remove the fiber background from the excitation fiber and to remove scattered laser light before the Raman signal reaches the collection fiber. An improved probehead based on this concept was later described in commonly assigned U.S. Pat. No. 5,377,004, which is incorporated herein by reference.
An additional feature of these probeheads involves the use of imaged excitation and collection. A block diagram of such a system is illustrated FIG. 1, along with the spectral content of the optical energy present at various points along the illumination and collection paths. Energy from an excitation laser 102 is coupled into the illumination fiber 104, beginning as a relatively pure, single wavelength of light as illustrated by the graph 106, which plots intensity as a function of wave number. Upon traversing fiber 104, the laser energy induces Raman scattering within the fiber material, typically composed of silica, yielding a spectrum 108 at the output of the illumination fiber which contains spurious Raman lines in addition to the laser wavelength.
Unless these undesired lines are eliminated from the illumination path before reaching the sample, their Rayleigh scatter at the sample may be indistinguishable from the true, shifted Raman scatter due to the laser excitation of the sample. Therefore, a laser band pass device 110 is used to remove these unwanted wavelengths, thereby outputting, ideally, the single laser line shown in graph 112 to the illumination optic 114 and sample under characterization 120 along path 116. It is assumed for the purposes of this discussion that illumination optic 114 contains a sufficiently short optical path that it does not itself generate significant spurious scattering.
The light scattered by sample 120, depicted by line 122, is collected by collection optic 124. At the output of collection optic 124, as depicted in graph 126, the scattered radiation consists of the unshifted Rayleigh scatter at the laser wavelength and the shifted Raman scatter that characterizes the sample 120 under test. Since the Rayleigh scatter is several orders of magnitude stronger than the Raman scatter, if allowed to enter collection fiber 130, this strong Rayleigh scatter can excite spurious Raman scattering within collection fiber 130 similar to this situation within illumination fiber 104.
This Rayleigh scatter must therefore be rejected before being coupled to collection fiber 130. This may be accomplished with a Rayleigh rejection element 132, which generates the spectra depicted in graph 134, now devoid of the strong Rayleigh line. The collection fiber 130 then conducts only the relatively weak Raman scattering lines, as depicted in graph 134, from sample 120 to an analysis instrument such as spectrograph 140 for detection.
Compared with the non-imaged approach of the McCreery-style probes, imaged excitation and collection as just described offer smaller sample measurement areas, sharper depth-of-focus, and a variable working distance between the probe and the sample. Imaging probeheads are, however, larger, more complex and more sensitive to alignment with the sample than non-imaging fiber optic Raman probes. In addition, imaging probeheads tend to have lower throughput than non-imaging fiber optic Raman probes, throughput being defined as Raman photons delivered to the spectrograph divided by the laser power incident on the excitation optical fiber.
There is, therefore, an unfulfilled need for a fiber optic based probe that combines the strengths of the McCreery-style probe (small size, high throughput, high-temperature operation, alignment insensitivity, easily mass-produced) with the strengths of the imaging probehead (no optical fiber background, variable working distance between probehead and sample)